Even the Evos have "full time" 4WD (as does the AYC in the rear), since besides the electronic pack, they still have a separate open diff setup, just like we do with the viscous coupling. Also, the front diff on an evo is in the transfer case with the center diff - the transmission has a single output.
The The GT-R has an electronic clutchpack that is decoupled from the drivetrain during normal operation until slipping occurs. The Haldex setups found on most Volkswagens and some Audis are also set up like this, except in a FWD configuration. (The Bugatti Veyron also uses a RWD Haldex setup - just much beefier and fancier.)
Porsche uses a setup with only a viscous coupling that is always rotating slightly and transferring a small amount of torque to the front (~5%), and no center diff. When the rear wheels slip, the coupling can transfer a fixed amount of torque to the front wheels, based on the rear slippage, but it reacts faster than a typical VCU because it's geared to already be spinning slightly. (Changing the tire diameters will actually mess this up)
It wasn't me that mentioned the Ralliart Evo I-III VCU - it was one of the NZ or Aussie guys. Alex at DSG said it was in his catalog for ~$800. Paired with a Cusco Tarmac gear, it could make for a pretty sweet RWD biased setup.
Quoting H05TYL:
No, the centre diff behaves just like an open diff when there is no wheelspin occuring, it drives all four wheels equally. When one pair of wheels starts turning faster than the other pair (front-rear) this causes the different speed of the output shafts from the centre diff (to both of which the viscous unit is attached) heats the fluid inside the viscous unit causing it to bind up and limit the amount of slippage.
Note that the lsd effect is initiated by difference in rotational speed , not torque.
Exactly the same principal as a viscous lsd in a rwd vehicle, it doesn't drive one wheel only until slippage occurs, and thereafter drive both.
All of the mitsubishi 4wds without active (electronic) centre differentials behave in this manner. (This is why the badge says, "Full time 4wd").
A possible source of the confusion is the Nissan Skyline GTR, which does not have a centre differential in the traditional sense. Instead it uses an electro hydraulically operated wet multi-plate clutch to engage drive to the front wheels. Under normal conditions the car is Rwd only, when the computer senses wheelspin it operates a hydraulic ram which engages the clutch plates. (This is located in the rear of the gearbox housing, to one side of the output shaft and is driven by chain).